Bias arrangement and apparatus

ABSTRACT

A biasing arrangement for an electronic apparatus having an input device is disclosed. The biasing arrangement comprises a bias voltage generator arranged to provide a bias voltage; a bias resistor connected between the bias voltage generator and the input device; and a reference voltage point of the input device, wherein a voltage of the reference voltage point of the input device is provided to the bias voltage generator for controlling the bias voltage in relation to the voltage of the reference voltage point of the input device. An electronic apparatus is also disclosed.

TECHNICAL FIELD

The present invention relates to a bias arrangement for an electronicapparatus having an input device, and such an apparatus.

BACKGROUND

Input devices, such as microphones or other devices providing an analogsignal sensitive to introduced noise, being connected by wire andrelying on an applied bias voltage may face electromagneticcompatibility problems due to loops being formed e.g. by multiplegrounding points. In the case of microphones, high currents to otherparts of a device may give rise to voltage fluctuations, which can bediscovered in the microphone signal as for example low frequency noise,often called “bumble bee” sound. Other problems that may arise in wiredconnections using a common reference for input and output devices can beecho effects. These effects normally have to be dealt with bywell-balanced designs of how wires and components are positioned andcircuitry and signal processing for attenuating known noise. WO02/056511 discloses an approach for dealing with such noise by enablinggeneration of a replica of periodic interference that is disturbing adesired signal so as to subtract out an estimation of the periodicinterference from the infected signal. The signal is bandpass filteredaround an expected frequency of the interfering component, and afundamental frequency thereof may thereafter be ascertained. Harmonicsof the fundamental frequency are generated so as to reconstruct anestimate of the interfering component in the frequency domain usingdetermined weights and Fourier series theory. The interfering componentestimate is subtracted to form the received signal to determine adesired component estimate. The weights may be optionally adjusted in afeedback loop.

However, there is also a desire to avoid that these interfering effectsoccur, or at least alleviate occurrence of them.

SUMMARY

The present invention is based on the understanding that at least someof the effects occur due to fluctuations in biasing of input devices,both in sense of a provided biasing voltage, but also in sense of areference voltage, such as ground or an intermediate reference voltagebetween a positive and a negative voltage level of a power supply. Thepresent invention is also based on the understanding that provision ofseveral voltage reference points forming conductive loops, which willcollect electromagnetic energy, contributes to introduction of noise.The inventors have found that by provision of a floating biasingarrangement, i.e. a biasing arrangement that do not rely on voltage of agrounding point which may provide such a conductive loop or which mayexperience a fluctuation in voltage relative to a grounding point of theinput device, the undesired effects can be alleviated.

According to a first aspect, there is provided a biasing arrangement foran electronic apparatus having an input device, the biasing arrangementcomprising a bias voltage generator arranged to provide a bias voltage;a bias resistor connected between the bias voltage generator and theinput device; and a reference voltage point of the input device, whereina voltage of the reference voltage point of the input device is providedto the bias voltage generator for controlling the bias voltage inrelation to the voltage of the reference voltage point of the inputdevice.

The reference voltage may be a ground voltage.

The bias voltage generator may comprise an amplifier having its outputconnected to the bias resistor, its positive input connected to thereference voltage point of the input device; a feedback resistorconnected between the output of the amplifier and a negative input ofthe amplifier; and a current generator connected to the negative inputof the amplifier and arranged to drive a current to a negative supplyvoltage.

The arrangement may comprise a signal output, wherein the biasingresistor and the input device are connected in series between the biasvoltage generator output and a reference voltage, and the signal outputis connected such that the alternating voltage over the biasing resistoris provided as an output signal.

The input device may comprise a field effect transistor having its drainand source connected to terminals of the analog input arrangement; and acapacitive sensor connected between gate and source of the field effecttransistor. The capacitive sensor may be any of a condenser microphone,a microelectromechanical system sensor, and a liquid capacitiveinclinometer.

The arrangement may further comprise a transistor connected between thebiasing resistor and the input device with the collector of thetransistor connected to the biasing resistor, the emitter of thetransistor connected to the input device, and the base connected to avoltage for providing a bias point such that a signal swing of theoutput signal at the connection to the biasing resistor is provided.

The signal output may comprise connections to respective terminals ofthe biasing resistor, and at least one of the connections comprises acapacitor for alternating current coupling of the output signal.

According to a second aspect, there is provided an electronic apparatushaving an input device, and a biasing arrangement, wherein the biasingarrangement comprises a bias voltage generator arranged to provide abias voltage; a bias resistor connected between the bias voltagegenerator and the input device; and a reference voltage point of theinput device, wherein a voltage of the reference voltage point of theinput device is provided to the bias voltage generator for controllingthe bias voltage in relation to the voltage of the reference voltagepoint of the input device.

The reference voltage may be a ground voltage.

The bias voltage generator may comprise an amplifier having its outputconnected to the bias resistor, its positive input connected to thereference voltage point of the input device; a feedback resistorconnected between the output of the amplifier and a negative input ofthe amplifier; and a current generator connected to the negative inputof the amplifier and arranged to drive a current to a negative supplyvoltage.

The apparatus may comprise a signal output, wherein the biasing resistorand the input device are connected in series between the bias voltagegenerator output and a reference voltage, and the signal output isconnected such that the alternating voltage over the biasing resistor isprovided as an output signal.

The input device may comprises a field effect transistor having itsdrain and source connected to terminals of the analog input arrangement;and a capacitive sensor connected between gate and source of the fieldeffect transistor. The capacitive sensor may be any of a condensermicrophone, a microelectromechanical system sensor, and a liquidcapacitive inclinometer.

The apparatus may further comprise a transistor connected between thebiasing resistor and the input device with the collector of thetransistor connected to the biasing resistor, the emitter of thetransistor connected to the input device, and the base connected to avoltage for providing a bias point such that a signal swing of theoutput signal at the connection to the biasing resistor is provided.

The signal output may comprise connections to respective terminals ofthe biasing resistor, and at least one of the connections comprises acapacitor for alternating current coupling of the output signal.

The input device of the apparatus may comprise a microphone.

The apparatus may comprise a wired connection to the input device whichcomprises an antenna, wherein the antenna is connected to a radioreceiver of the apparatus and separated from an audio input circuitry ofthe apparatus by a choke coil. The apparatus may further comprising anoutput device, wherein the reference voltage point is used as referencevoltage also for an output device. The output device of the apparatusmay comprise at least one speaker. The wired connection to the inputdevice may comprise a connector for detaching the input device from theapparatus.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically illustrating a prior art biasingarrangement.

FIG. 2 is a diagram schematically illustrating a biasing arrangementaccording to an embodiment.

FIG. 3 is a diagram schematically illustrating a biasing arrangementaccording to an embodiment.

FIG. 4 is a diagram schematically illustrating parts of an electronicapparatus according to an embodiment.

FIG. 5 illustrates a bias voltage generator according to an embodiment.

FIGS. 6 to 11 schematically illustrates examples of suitable currentgenerators arranged to provide a constant current.

FIG. 12 schematically illustrates a voltage generator suitable forgeneration of a constant voltage.

FIG. 13 schematically illustrates that a voltage divider of a currentgenerator is fed by another current generator to provide the definedvoltage to e.g. a base of a transistor, which thereby provides aconstant current I.

FIG. 14 is a diagram schematically illustrating an exemplary inputdevice.

FIG. 15 is a diagram schematically illustrating a circuit arrangementaccording to an embodiment.

FIG. 16 illustrates an electronic apparatus according to an embodiment.

DETAILED DESCRIPTION

For biasing, a biasing resistor can be connected to a supply voltage andto an analog input arrangement. The analog input arrangement istraditionally connected to a reference voltage, e.g. ground, such thatbiasing resistor and the analog input arrangement are connected inseries between the supply voltage and the reference voltage. The signalover the analog input arrangement is traditionally connected by a signaloutput to provide an output signal. The signal output preferablycomprises a capacitor for alternating current (AC) coupling of theoutput signal.

FIG. 1 is a diagram schematically illustrating a prior art biasingarrangement 100. The biasing arrangement 100 is applicable e.g. for anelectronic apparatus 101 having an input device 102 connected by wires103, 104 to signal processing circuitry 105 of the electronic apparatus101. The biasing arrangement 100 comprises a bias voltage generator 106arranged to provide a bias voltage Vbias. A reference voltage Vref canbe ground or an intermediate reference voltage between a positive and anegative voltage level of a power supply. The bias voltage Vbias is avoltage assigned to the input device 102 to work properly. Thus, thevoltage over the input device 102 will be the voltage difference betweenthe reference voltage Vref and a portion of the bias voltage Vbias,which is divided between the input device 102 and a biasing resistor 107to enable a suitable swing of the input device 102.

The biasing arrangement 100, which is a part of the electronic apparatus101, and other circuitry of the electronic apparatus, such as the signalprocessing circuitry 105, normally has a voltage reference, such asground 108. The input device 102 is also connected to a referencevoltage 109, which is preferred to have the same voltage as the otherreference voltage 108 of the electronic apparatus 101. The referencevoltage 109, which preferably is arranged close to the input device 102,will give proper protection against electrostatic discharges. However,due to different physical locations of reference voltage points 108,109, 110, there can be a difference in voltage, normally an alternatingdifference, which can be modelled as an alternating current voltagesource 111 between the reference voltage points 108, 110 and 109,respectively. This will add noise to the signal provided to the signalprocessing circuitry 105 from the input device 102.

The present invention alleviates this problem by providing a floatingbias generator. This can be implemented according to the principlesillustrated in FIG. 2, which for illustrative purposes is made assimilar as possible to the prior art solution illustrated in FIG. 1.

In FIG. 2, there is illustrated a biasing arrangement 200 of anelectronic apparatus 201 having an input device 202 connected by wires203, 204 to signal processing circuitry 205 of the electronic apparatus201. Similar to the prior art solution demonstrated with reference toFIG. 1, the biasing arrangement 200 comprises a bias voltage generator206 arranged to provide a bias voltage Vbias, and a biasing resistor207, but here the biasing arrangement 200 is made floating from areference voltage 208 of the electronic apparatus 201. Instead, from areference voltage 209 of the input device 202, there is a connection 210to the bias voltage generator 206 such that the bias voltage generator206 can be able to provide the bias voltage Vbias in relation to theactual reference voltage 209 of the input device 202. Thus, noise addedby a modelled alternating current voltage source 211 between thereference voltage points 208 and 209 will not add noise in the same wayto the signal provided to the signal processing circuitry 205 from theinput device 202 since common mode rejection of the signal processingcircuitry 205 will be able to handle this. It is preferable that thesignal processing circuitry 205 has sufficient input impedance for thiscommon mode rejection, e.g. at least 100 kΩ.

FIG. 3 schematically illustrates an electronic apparatus 300 similar tothe one illustrated in FIG. 2, but with the difference that the signalis taken across the bias resistor 302. This configuration provides for afurther rejection of noise, here from any noise present in the biasvoltage. A noise rejection of typically 26 dB can be achieved. Asubstantive common mode rejection can also be achieved. Considering acase where the input device 304 has an analog sensor element and a fieldeffect transistor (FET) as schematically illustrated in FIG. 14, wherethe FET has its drain and source connected to the terminals of the inputdevice 304, a further improvement is also at option. The analog sensorelement can be a capacitive sensor connected between gate and source ofthe FET. The capacitive sensor can for example be a condensermicrophone, a microelectromechanical system sensor, or a liquidcapacitive inclinometer. Upon registration of a physical phenomenon,such as sound, force or inclination, the capacitance of the analogsensor element changes. Thus, an applied charge, which is provided bythe biasing arrangement as demonstrated above, on the analog sensorelement causes the voltage over the input device 304 to change. Theinput device can also be an electronic circuit or device providing inputover the gate and source of the field effect transistor. The furtheradvantage can be achieved by arranging a transistor, as schematicallyillustrated in FIG. 15, connected between the biasing resistor 302 andthe input device 304 with the collector of the transistor connected tothe biasing resistor 302, the emitter of the transistor connected to theinput device 304, such that the biasing resistor 302, the transistor andthe input device 304 are connected in series between the bias voltagegenerator 306 and the reference voltage 309. The base of the transistoris connected to a voltage, e.g. provided by a voltage divider, forproviding a bias point such that a signal swing of the output signal atthe connection to the biasing resistor 302 is provided. By thisconfiguration and considering the configuration having the transistorand the FET, it can be seen that the transistor and the FET will be incascode, which has been found to be beneficial. The signal of the inputdevice 304 is no longer superposed on its driving voltage. If the baseof the transistor is properly connected to a low impedance, thetransistor will work as an emitter follower, which has low impedance atits emitter, which provides the driving voltage to the input device 304.The signal from the input device 304 will be present at the collector ofthe transistor, and by the properly applied voltage to the base, thecollector will swing to provide the output signal across the biasingresistor 302. The voltage to the base of the transistor can becontrolled to achieve further effects, such as adapting sensitivity ofthe analog input arrangement. Preferably, the transistor is a bipolarnpn transistor, which is particularly beneficial since it provides lowoutput impedance and good control properties regarding voltage appliedon the base. However, if high power is to be output or for otherreasons, a more robust transistor or another suitable active device suchas a MOSFET can be used.

FIG. 4 is a diagram schematically illustrating parts of an electronicapparatus 400 including a biasing arrangement 402 according to anembodiment. The apparatus has an input device 404, e.g. a microphone,connected by wires 405, 406. The biasing arrangement 402 comprises abias signal generator 408 arranged to provide a bias voltage Vbias,similar to what is described with reference to FIG. 2. The biasingarrangement 402 thus further comprises a bias reference connection to aground point 406 of the input device 404, i.e. a ground point located asclose as possible and connected to the input device, which biasreference is used by the bias signal generator 408 to provide the biasvoltage in relation to the bias reference. The wired connection canfurther comprise a filter for filtering the bias voltage. The filter canbe a low-pass filter.

The wired connection to the input device can further comprise anantenna, wherein the antenna is connected to a radio receiver 424 of theapparatus 400 and separated from an audio input circuitry 426 of theapparatus 400 by a choke coil 428. The apparatus 400 can furthercomprise an output device 430, e.g. one or more speakers, which is alsoconnected by wires 406, 407, wherein the ground point 417 is used asreference voltage also for the output device 430. The output device 430is for example driven by an output signal circuitry 432, which has itsoutput connected to the wire 407. The wired connection to the inputdevice 404, and preferably also the output device 430, can comprise aconnector 434 for detaching the input device 404, and preferably alsothe output device 430, from the apparatus 400, as will be furtherillustrated below with reference to FIG. 16. The bias voltage can begenerated by the bias signal generator 408 by observing the biasreference signal 436 such that a feedback loop is provided. The set-upcan be modified as demonstrated with reference to FIGS. 3, 14 and 15,with different ways of taking out the desired signal, as well as withadded circuitry for further noise handling. An embodiment of this willbe demonstrated with reference to FIG. 5 below.

FIG. 5 illustrates a bias voltage generator 500 connected to a biasresistor 502 and an input device 504. The set-up can be modified asdemonstrated with reference to FIGS. 3, 14 and 15, with different waysof taking out the desired signal, as well as with added circuitry forfurther noise handling. The bias voltage generator comprises anamplifier 506, preferably an operational amplifier, supplied with apositive and a negative power supply voltages +V, −V. A negativefeedback is provided by a resistor R₂, which is also connected to thenegative power supply voltage −V via a current generator 508, which canbe of any suitable type, e.g. as any of the examples that will be givenwith reference to FIGS. 6 to 13. A bias reference signal taken from agrounding point 509 of the input device 504 is connected to the positiveinput of the amplifier 506. The amplifier will, though its feedback,make its negative and positive inputs to have the same potential, whichis achieved when the voltage over the feedback resistor R₂ is thepotential difference between V_(bias) and the bias reference. Since thecurrent generator 508 will provide a constant current I, which inpractice also will flow through the feedback resistor R₂, the potentialdifference between V_(bias) and the bias reference will always be R₂I,which thus will be provided over the bias resistor 502 and the inputdevice 504. A floating and correct bias will thus be provided,irrespective of other grounding points of an apparatus in which thearrangement is provided, and noise problems as those demonstrated withreference to FIG. 1 will be alleviated.

FIGS. 6 to 13 schematically illustrates examples of suitable currentgenerators arranged to provide a constant current, which can be used inany of the embodiments demonstrated with reference to FIG. 5.

The example illustrated in FIG. 6 relies on a zener diode tuned by acapacitor for noise reduction, which zener diode provides a definedvoltage to a base of a transistor, which thereby provides a constantcurrent I.

The example illustrated in FIG. 7 relies on a voltage divider whichprovides a defined voltage to a base of a transistor, which therebyprovides a constant current I.

The example illustrated in FIG. 8 relies on a voltage divider whichprovides a defined voltage to a base of a transistor, which therebyprovides a constant current I. The voltage divider is further providedwith a diode for compensating voltage over base and emitter of thetransistor.

The example illustrated in FIG. 9 relies on a diode for compensatingvoltage over base and emitter of the transistor and a further diode fordefining a voltage to a base of a transistor, which thereby provides aconstant current I.

The example illustrated in FIG. 10 relies on a voltage divider whichprovides a defined voltage to a darlington pair, which thereby providesa constant current I.

The example illustrated in FIG. 11 relies on a junction field effecttransistor with a source resistor arranged to provide a gate to sourcevoltage, which thereby provides a constant current I.

The example illustrated in FIG. 12 relies on a zener diode filtered by acapacitor for noise reduction, which zener diode provides a definedvoltage to a positive input of an amplifier via a low-pass filter. Theamplifier, which can be an operational amplifier, is provided with afeedback from its output to its negative input, and thereby provides aconstant voltage, which can be used for bias generation. This can inturn be used for generation of a constant current, e.g. by providing theconstant voltage to a base of a transistor, for example directly or viaa voltage divider.

The example illustrated in FIG. 13 illustrates that a voltage divider ofa current generator, for example as the one illustrated in FIG. 7, isfed by another current generator, for example as the one illustrated inFIG. 11, to provide the defined voltage to e.g. a base of a transistor,which thereby provides a constant current I. Any of the currentgenerators illustrated with reference to FIGS. 6 to 10, and 12 relyingon a provided voltage can be used, and any of the current generatorsillustrated with reference to FIGS. 6 to 11 can be used to provide thecurrent to the voltage divider, respectively.

Any unwanted AC components of the supply voltage provided by the biasingarrangement, which ideally should have pure direct current (DC)properties, may propagate through the series coupling, i.e. the inputdevice and the biasing resistor, and the signal output and add noise tothe output signal. Below, a few approaches how to cope with suchunwanted AC components will be demonstrated.

FIG. 14 is a diagram schematically illustrating an exemplary inputdevice 1400 with a sensor element 1402 and a field effect transistor(FET) 1404. The FET has its drain and source connected to terminals1406, 1408 of the input device 1400. The sensor element 1402 ispreferably a capacitive sensor connected between gate and source of theFET 1404. The capacitive sensor can for example be a condensermicrophone, a microelectromechanical system sensor, or a liquidcapacitive inclinometer. Upon registration of a physical phenomenon,such as sound, force or inclination, the capacitance of the sensorelement 1402 changes. Thus, an applied charge, which is provided by thebiasing arrangement as demonstrated above, on the sensor element 1402causes the voltage over the sensor element 1402 to change. The sensorelement 1402 can also be an electronic circuit or device providing inputover the gate and source of the field effect transistor.

FIG. 15 is a diagram schematically illustrating a circuit arrangement1500 according to an embodiment. The circuit arrangement 1500 comprisesa biasing arrangement 1502. The biasing arrangement 1502 comprises abiasing resistor connected to a bias voltage 1504, which is providedaccording to any of the embodiments demonstrated above. An input device1506 is connected to a reference voltage 1508, e.g. a ground point invicinity of the input device 1506. A transistor 1510 is connectedbetween the biasing resistor 1502 and the input device 1506 with thecollector of the transistor 1510 connected to the biasing resistor 1502,the emitter of the transistor connected to the input device 1506, suchthat the biasing arrangement 1502, the transistor 1510 and the inputdevice 1506 are connected in series between the supply voltage 1504 andthe reference voltage 1508. The signal over the biasing resistor 1502 isconnected by a signal output 1512 to provide an output signal from thecircuit arrangement 1500. The signal output 1512 preferably comprisescapacitors 1514, 1516 for AC coupling of the output signal. The base ofthe transistor 1510 is connected to a voltage, e.g. provided by avoltage divider 1518, 1520, for providing a bias point such that asignal swing of the output signal at the connection to the biasingresistor 1502 is provided.

In the configuration of FIG. 15, considering the configuration of theinput device 1400 demonstrated with reference to FIG. 14, the transistor1510 and the FET 1404 will be in cascode, which has been found to bebeneficial. The signal of the analog input arrangement 1506 is no longersuperposed on its driving voltage. If the base of the transistor 1510 isproperly connected to a low impedance 1520, the transistor 1510 willwork as an emitter follower, which has low impedance at its emitter,which provides the driving voltage to the analog input arrangement 1506.The signal from the analog input arrangement 1506 will be present at thecollector of the transistor 1510, and by the properly applied voltage tothe base, the collector will swing to provide the output signal acrossthe biasing resistor 1502. The voltage to the base of the transistor1510 can be controlled to achieve further effects, such as adaptingsensitivity of the analog input arrangement. In the illustration of FIG.15, the transistor 1510 is a bipolar npn transistor, which isparticularly beneficial since it provides low output impedance and goodcontrol properties regarding voltage applied on the base. However, ifhigh power is to be output or for other reasons, a more robusttransistor or another suitable active device such as a MOSFET can beused.

The approach is based on the understanding that at least some of theeffects occur due to fluctuations in biasing of input devices in senseof a provided biasing voltage. The approach is also based on theunderstanding that provision of the analog signal output to anamplifier, taken over the high impedance field effect transistorcontributes to taking up a considerable amount of the noise, while ananalog signal taken over a series resistor, which is also used for thebiasing and having a considerable lower impedance than the arrangementwith the analog input and the field effect transistor, reduces the noisecomponent provided to the amplifier. The approach can be used as asubstitute, or a complement, to the filter demonstrated with referenceto FIG. 4.

For the embodiments of the approach demonstrated with reference to FIG.3, i.e. getting the signal over a biasing resistor, the scheme of FIG. 4will be slightly modified in the connection of the signal to the audioinput circuit. This modification has been elucidated with reference toFIG. 3, and therefore, no separate scheme illustrating this embodimentof the electronic apparatus is considered necessary for a person skilledin the art to be able to do the modification.

The configuration of FIG. 15 implies that inherent properties of theinput input device 1506, as demonstrated with reference to FIG. 14,provides for a rejection of noise present in the bias voltage 1504. Anoise rejection of typically 26 dB can be achieved. A substantive commonmode rejection can also be achieved.

The approach demonstrated in FIG. 3 and corresponding suggestedmodifications is based on the understanding that at least some of theeffects occur due to fluctuations in biasing of input devices in senseof a provided biasing voltage. The approach is also based on theunderstanding that provision of the analog signal output to anamplifier, taken over the high impedance field effect transistorcontributes to taking up a considerable amount of the noise, while ananalog signal taken over a series resistor, which is also used for thebiasing and having a considerable lower impedance than the arrangementwith the analog input and the field effect transistor, reduces the noisecomponent provided to the amplifier. The approach can be used as asubstitute, or a complement, to the filter demonstrated with referenceto FIG. 4.

FIG. 16 illustrates an electronic apparatus 1600 according to anembodiment, where the principles of the invention are applicable. Theapparatus 1600 can represent an example of the apparatus 400 illustratedby the schematic diagram of FIG. 4. The electronic apparatus 1600 canfor example be a mobile phone or a media player/recorder. The apparatus1600 comprises a main body 1602 having power source, circuitry, userinterface, etc. The user interface can comprise a keypad 1604, a display1606, a microphone 1608 and a speaker 1610. The apparatus also comprisesa connector 1612 to which an external microphone 1614 and externalspeakers 1616, 1616′, e.g. forming a headset 1618, can be connected by aconnector 1620 of the headset 1618. The connectors 1612, 1620 arearranged to mate each other and provide electrical contacts between themain body circuitry and the input and output devices 1614, 1616, 1616′of the headset 1618 for input and output of signals. The connectors1612, 1620 can thus be connected and disconnected to each other, whichmay be facilitated by grippable surfaces 1622 of the connector 1620 ofthe headset 1618. A wire of a cord 1624 of the headset 1618 can alsowork as a radio antenna for the apparatus 1600, e.g. in the FM bandbetween 87.5-108.0 MHz, as demonstrated above with reference to FIG. 4.

1. A biasing arrangement for an electronic apparatus having an input device, the biasing arrangement comprising a bias voltage generator arranged to provide a bias voltage; a bias resistor connected between the bias voltage generator and the input device; and a reference voltage point of the input device, wherein a voltage of the reference voltage point of the input device is provided to the bias voltage generator for controlling the bias voltage in relation to the voltage of the reference voltage point of the input device.
 2. The arrangement according to claim 1, wherein the reference voltage is a ground voltage.
 3. The arrangement according to claim 1, wherein the bias voltage generator comprises an amplifier having its output connected to the bias resistor, its positive input connected to the reference voltage point of the input device; a feedback resistor connected between the output of the amplifier and a negative input of the amplifier; and a current generator connected to the negative input of the amplifier and arranged to drive a current to a negative supply voltage.
 4. The arrangement according to claim 3, comprising a signal output, wherein the biasing resistor and the input device are connected in series between the bias voltage generator output and a reference voltage, and the signal output is connected such that the alternating voltage over the biasing resistor is provided as an output signal.
 5. The arrangement according to claim 1, wherein the input device comprises a field effect transistor having its drain and source connected to terminals of the analog input arrangement; and a capacitive sensor connected between gate and source of the field effect transistor.
 6. The arrangement according to claim 5, wherein the capacitive sensor is any of a condenser microphone, a microelectromechanical system sensor, and a liquid capacitive inclinometer.
 7. The arrangement according to claim 4, further comprising a transistor connected between the biasing resistor and the input device with the collector of the transistor connected to the biasing resistor, the emitter of the transistor connected to the input device, and the base connected to a voltage for providing a bias point such that a signal swing of the output signal at the connection to the biasing resistor is provided.
 8. The arrangement according to claim 4, wherein the signal output comprises connections to respective terminals of the biasing resistor, and at least one of the connections comprises a capacitor for alternating current coupling of the output signal.
 9. An electronic apparatus having an input device, and a biasing arrangement, wherein the biasing arrangement comprises a bias voltage generator arranged to provide a bias voltage; a bias resistor connected between the bias voltage generator and the input device; and a reference voltage point of the input device, wherein a voltage of the reference voltage point of the input device is provided to the bias voltage generator for controlling the bias voltage in relation to the voltage of the reference voltage point of the input device.
 10. The apparatus according to claim 9, wherein the reference voltage is a ground voltage.
 11. The apparatus according to claim 9, wherein the bias voltage generator comprises an amplifier having its output connected to the bias resistor, its positive input connected to the reference voltage point of the input device; a feedback resistor connected between the output of the amplifier and a negative input of the amplifier; and a current generator connected to the negative input of the amplifier and arranged to drive a current to a negative supply voltage.
 12. The apparatus according to claim 11, comprising a signal output, wherein the biasing resistor and the input device are connected in series between the bias voltage generator output and a reference voltage, and the signal output is connected such that the alternating voltage over the biasing resistor is provided as an output signal.
 13. The apparatus according to claim 9, wherein the input device comprises a field effect transistor having its drain and source connected to terminals of the analog input arrangement; and a capacitive sensor connected between gate and source of the field effect transistor.
 14. The apparatus according to claim 13, wherein the capacitive sensor is any of a condenser microphone, a microelectromechanical system sensor, and a liquid capacitive inclinometer.
 15. The apparatus according to claim 12, further comprising a transistor connected between the biasing resistor and the input device with the collector of the transistor connected to the biasing resistor, the emitter of the transistor connected to the input device, and the base connected to a voltage for providing a bias point such that a signal swing of the output signal at the connection to the biasing resistor is provided.
 16. The apparatus according to claim 12, wherein the signal output comprises connections to respective terminals of the biasing resistor, and at least one of the connections comprises a capacitor for alternating current coupling of the output signal.
 17. The apparatus according to claim 9, wherein the input device of the apparatus comprises a microphone.
 18. The apparatus according to claim 9, comprising a wired connection to the input device which comprises an antenna, wherein the antenna is connected to a radio receiver of the apparatus and separated from an audio input circuitry of the apparatus by a choke coil.
 19. The apparatus according to claim 9, further comprising an output device, wherein the reference voltage point is used as reference voltage also for an output device.
 20. The apparatus according to claim 19, wherein the output device of the apparatus comprises at least one speaker.
 21. The apparatus according to claim 18, wherein the wired connection to the input device comprises a connector for detaching the input device from the apparatus. 